This invention relates to weldable pipe fittings, by which is meant hollow thermoplastic fittings connectable to thermoplastic pipes in which a part of an inner peripheral wall of the fitting is arranged to be joined by fusion or "welding" to a part of an outside peripheral wall of a plastics pipe. The invention relates more particularly to the production of such weldable fittings adapted to be fused to the outside periphery of a pipe by means of an electric heating wire embedded within the fitting.
It is to be understood that the expression "pipe" as used herein includes pipes and tubes as such and also pipe-like members and fittings for use in pipework such as bends, elbows, "T" connectors, flange adaptors and parts and devices adapted to be fitted by pipe-like parts to the pipework such as valves and pumps.
It has already been proposed to provide a hollow plastics fitting of the kind referred to in which a helical groove is formed around a part of the inner wall of a sleeve-like part of a fitting within which is located an electrically conducting wire, so that when in use the fitting is placed in close proximity about a thermoplastic pipe or pipe-like member and an electric current passed through the electic wiring, melting of the adjacent plastics surfaces of the fitting and the pipe occurs and fusion or "welding" between the two bodies takes place.
Such arrangements are disclosed in our United Kingdom Patents 2135746B and 2135747B.
One of the critical factors in design of such fittings is ensuring the provision of consistently successful welds meeting professionally specified performance requirements. A most important aspect in this is achieving the correct conditions at the welding interface between the fitting and the pipe. Two such conditions comprise obtaining a suitable heating profile for satisfactory welding, and, perhaps more difficult to achieve, correct pressure within the molten plastics material during the welding process.
With the development of large diameter fittings for large diameter pipes, of 180 mm above, for example, the performance pressure requirements necessitate the use of thicker walled fittings and from this the achievement of the correct amount of heat and pressure during welding presents increasingly difficult problems with increase in size.
Thus, for example, the thickened walls of such large fittings can cause excessive melt pressure during the welding process resulting in molten polymer, and sometimes part of the heating wires, being extruded from between the pipe and the pipe fitting. Such movement of heating wires can cause, in extreme cases, overheating by contacting and short circuiting with each other, whilst at the other extreme, premature joint failure can be caused due to the chilling effect the large cold pipe has on the melt, resulting in crack propagation and joint failure.
Although theoretically a reduction in pressure at the weld melt could be achieved by reducing the wall thickness of the fitting, this can seriously compromise the stress performance under pressure in use of the fitting, whilst, in some situations still leading to exudation of molten polymer, and even at times voids along the weld interface due to the melt pressure producing a radially outward bending along the length of the reduced thickness and weakened fitting.
It is an object of the present invention to overcome or at least substantially reduce the above mentioned difficulties and problems.